In recent years, vibrators for use as time standards for electronic devices have started to be expected so that their vibrators are of small sizes and have a small CI-value (crystal impedance or equivalent series resistance). Conventionally, known vibrators that meet this requirement, e.g., tuning fork-type crystal vibrators, include one that is constructed in the manner shown in FIGS. 7A and 7B (see Japanese Patent Application Laid-Open No. 2002-76806, for example).
The tuning fork-type crystal vibrator described in the above patent document has two vibrating tines 13 that constitute a tuning fork, and groove portions 9 or slots are formed on the respective obverse and reverse surfaces of the tines 13. As shown in FIG. 7B, each tine 13 has a substantially H-shaped cross section. Drive electrodes are formed on the respective inner wall surfaces of the groove portions 9. This tuning fork-type crystal vibrator having a substantially H-shaped cross section is characterized in that the CI-value can be restricted to a low range, since its electromechanical conversion factor can be enhanced despite its size smaller than that of a conventional vibrator.
Processes for the manufacture of the tuning fork-type crystal vibrator with a substantially H-shaped cross section will now be described with reference to FIGS. 8A to 10D (see Japanese Patent Application Laid-Open No. 2002-76806 described above).
First, a crystal substrate 1 is worked into a plate-like structure, as shown in FIG. 8A. Then, metal films, a Cr film 3 and an Au film 5, are formed on the obverse and reverse surfaces of the crystal substrate 1 by sputtering (FIG. 8B). A photoresist layer 7 is formed on the metal films formed in this manner (FIG. 8C). Then, the external shape of the tuning fork-type crystal vibrator is exposed and developed with use of a photomask, and patterning is performed so that the photoresist layer 7 remains on the inside of the external shape of the tuning fork-type crystal vibrator and that the outside metal film, an unnecessary part, is exposed (FIG. 8D). FIG. 8D is a sectional view showing portions corresponding to the vibrating tines 13 of the tuning fork-type crystal vibrator.
Then, the exposed metal films, the Au film 5 and the Cr film 3, are etched away in the order named (FIG. 9A). After the entire remaining photoresist layer 7 is then exfoliated (FIG. 9B), a photoresist is applied again to the entire surface of the crystal substrate 1, whereupon a new photoresist layer 7 is formed (FIG. 9C). Then, the new photoresist layer 7 is exposed to the external shape of the tuning fork-type crystal vibrator and the shape of the groove portions 9 of the vibrating tines 13 with use of a photomask and developed, whereupon the surface of the unnecessary part of the crystal substrate 1 outside the external shape of the tuning fork-type crystal vibrator and the metal films of the groove portions 9 are exposed (FIG. 9D).
Then, the crystal substrate 1 that is exposed with the etchant for crystal etching is etched. As a result of the etching, the external shape of the tuning fork-type crystal vibrator is formed (FIG. 10A). Subsequently, the metal films (Au film 5 and Cr film 3) are etched with the remaining photoresist layer 7 used as a mask, and the metal films, the Au film 5 and the Cr film 3, exposed in the groove portions are removed in the order named (FIG. 10B). Then, the crystal substrate 1 that is exposed corresponding to the groove portions 9 is etched to a predetermined depth with the etchant for crystal etching, whereupon the groove portions 9 are formed (FIG. 10C). Then, the finally remaining photoresist layer 7 and the metal films are removed, whereupon the shape of the tuning fork-type crystal vibrator with a substantially H-shaped cross section is completed (FIG. 10D). Thereafter, electrodes are formed on the vibrator shown in FIG. 10D, whereupon the tuning fork-type crystal vibrator with a substantially H-shaped cross section is completed.
In the manufacturing method described above, however, the photoresist layer 7 must be formed and exfoliated twice, so that more complicated processes are required than in a manufacturing method for an ordinary tuning fork-type crystal vibrator without any grooves in its cross section, and the operating efficiency is poor. Further, the secondarily formed photoresist layer 7 (FIG. 9C) is exposed again for the external shape with use of the photomask for the formation of the groove portions different from the photomask for the primarily formed external shape. Accordingly, there is a problem that misalignment is caused between the primarily formed metal films (Au film 5 and Cr film 3) and the new photoresist film. Thus, a technique to improve this problem is proposed in Japanese Patent Application Laid-Open No. 2002-261557.
Disclosed in this Japanese Patent Application Laid-Open No. 2002-261557 is a manufacturing method for a vibrating piece of an inverted-mesa AT-cut crystal. Referring now to FIGS. 11A to 13B, a case will be described in which this technique is applied to a tuning fork-type crystal vibrator in comparison with the present invention.
First, a crystal substrate 1 is prepared (FIG. 11A), and a Cr film 3 and an Au film 5 are formed on the obverse and reverse surfaces of the crystal substrate 1 by vapor deposition or sputtering (FIG. 11B). Then, a photoresist is applied to the surfaces of these metal films (Cr film 3 and Au film 5) to form a photoresist layer 7 (FIG. 11C). Then, the external shape of the tuning fork-type crystal vibrator is exposed and developed with use of a photomask, and patterning is performed so that the photoresist layer 7 remains on the inside of the external shape of the tuning fork-type crystal vibrator and that the outside metal film is exposed (FIG. 11D).
Then, the exposed metal films, the Au film 5 and the Cr film 3, are etched away in the order named (FIG. 12A). Subsequently, the remaining photoresist layer 7 is exposed again for the shape of the groove portions 9 with use of a photomask and developed, whereupon the metal films of the groove portions are exposed (FIG. 12B). Then, the exposed crystal substrate 1 is etched with the etchant for crystal etching. The external shape of the tuning fork-type crystal vibrator is formed as a result of the etching (FIG. 12C). Subsequently, the metal films are etched with the remaining photoresist layer 7 as a mask, and the metal films, the Au film 5 and the Cr film 3, exposed in the groove portions are removed in the order named (FIG. 12D).
Then, the crystal substrate 1 that is exposed corresponding to the groove portions is etched to a predetermined depth with the etchant for crystal etching, whereupon the groove portions 9 are formed (FIG. 13A). Then, the remaining photoresist layer 7 and the metal films are removed, whereupon the shape of the tuning fork-type crystal vibrator with a substantially H-shaped cross section is completed (FIG. 13B). Thereafter, electrodes (not shown) are formed on this crystal vibrator, whereupon the tuning fork-type crystal vibrator with a substantially H-shaped cross section is completed.
When the photoresist layer is exposed to the etchant for the metal films, according to the manufacturing method described above, the surface of the photoresist layer is degenerated to lower the exposure sensitivity, and a degenerated-surface layer is produced that cannot be dissolved by normal exposure and development. Proposed in the aforesaid Japanese Patent Application Laid-Open No. 2002-261557, therefore, is a method in which the exposure time for the exposure of the groove portions is extended, the degenerated-surface layer is removed by an alkaline solution, such as a developer, before the exposure of the groove portions, or the degenerated-surface layer is removed by dry etching using oxygen plasma before the exposure of the groove portions or before the development after the exposure.
However, the alternative prior art manufacturing method described above has the following problems. Since the photoresist layer is degenerated by the etchant for removing the metal films, the photoresist must be subjected to some treatment. There is a problem that the degenerated-surface layer of the photoresist layer 7 cannot be developed if an attempt is made to extend or even decouple the exposure time in order to compensate for the reduction of the exposure sensitivity of the degenerated-surface layer. If the exposure is prolonged, moreover, the exposure time is too long for the photoresist at an undegenerated region, so that the dimensional accuracy of patterns worsens inevitably.
If an attempt is made to remove the degenerated-surface layer of the photoresist layer by an alkaline solution, such as a developer, before the exposure, moreover, the degenerated-surface layer is hardly dissoluble in the alkaline solution, so that it is difficult to remove the degenerated-surface layer thoroughly.
According to the method in which the degenerated-surface layer is removed by dry etching using oxygen plasma, furthermore, the processes are complicated, and besides, the photoresist layer is unexpectedly exposed to ultraviolet rays generated from the oxygen plasma, so that the pattern accuracy of the groove portions 9 worsens.